RU2494165C2 - High-strength high-ductility steel rod and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel rod is subjected to finish rolling, steel having the following composition in wt %: 0.07 - 0.14% of aluminium (Al), and nitrogen (N), wherein Al:N, where Al and N stand for wt % of every element and make 15:1 - 25:1 at 850°C to 1050°C. Steel is cooled at the rate of about 5°C/s or lower for making the rod wherein AlN-based nano-inclusions are formed sized to about 130 nm or smaller. Sais rod can be produced using simple composition of alloys without additional thermal treatment.

EFFECT: improved strength and ductility.

7 cl, 8 dwg, 2 tbl, 1 ex

Description

FIELD OF THE INVENTION

The present invention relates to a high strength, highly viscous thin steel bar and a method for manufacturing it, and more particularly, to a high strength, high viscosity thin steel bar as a thin steel bar used, for example, for cold stamping, which can be manufactured in a simple manner compared to conventional not tempered by a steel bar, as well as using fewer alloying elements and with greater dimensional accuracy than a tempered thin steel bar obtained by ordinary m heat treatment method.

State of the art

Steel used for a thin bar requiring high strength and toughness, in particular for a thin steel bar for cold stamping, such as the S45C bar having a cold strength of 700 MPa or more, is a steel requiring ductility and toughness, necessary for processing, as well as requiring high strength in terms of the strength of the finished part. In other words, since the thin rod must be shaped into the finished product, it must have sufficient ductility and toughness so as not to undergo destruction during processing. After processing in the form of parts, a thin bar should help to reduce the weight of the device in which such parts, for example, a car, are used, with sufficient strength.

However, in general, the strength and toughness (viscosity as such) of the material are difficult to match, which is the reason for the phenomenon in which the viscosity decreases with increasing strength. Therefore, an example illustrating a conventional method for manufacturing a thin bar for cold stamping includes, first of all, performing spheroidizing annealing to ensure the viscosity and ductility of the thin bar, and then processing to obtain the desired shape of the part. Then hardening and tempering are carried out to give strength to the part. The steel (thin bar) obtained in this way is called “tempered steel", and as a method for its production, a method was proposed that includes rolling and hardening a steel bar, whereby an elongated bar is obtained as a result of processing the martensite converted during the process, while maintaining in the temperature range from 500 ° C to 600 ° C.

However, there is a limitation regarding a possible increase in production cost, since in order to improve hardenability, as well as to carry out additional processes, such as hardening and tempering, in order to give steel sufficient strength as a result of quenching and tempering, it is necessary to introduce a large number of alloying elements. In addition, the dimensional accuracy of the part is deteriorated due to thermal stress arising in the part as a result of the hardening process. The cost of production equipment also increases, so when getting a thin steel bar in this way, overcoming the limitations associated with an increase in cooling speeds and pressure during rolling becomes difficult.

To overcome the restrictions on tempered steel, non-tempered steel has been developed that is not heat-treated for tempering. The term "non-tempered steel" means steel, the desired level of strength of which can be obtained without one or more stages of heat treatment, such as spheroidizing annealing or hardening and tempering. However, since the treatment is usually carried out in a state satisfying the strength required for the final product, high strain resistance occurs during the processing of the thin steel bar. Therefore, in many cases, for parts requiring relatively low machinability, such as a pin or U-bolt, non-tempered steel is used.

One of the methods for producing non-tempered steel may include obtaining a microstructure having high strength and toughness in steel by controlling the composition of the alloying elements and the cooling mode. According to this method, a microstructure having high strength and toughness is selected among the internal microstructures of the steel using the number of alloying components and a cooling model suitable for the given microstructure. For example, a composition in which the C content, which is undesirable from the point of view of toughness, and there is a mixture of elements such as Cr and Mo, is used as an appropriate alloying composition. The steel obtained by the alloying composition is rolled, after which the microstructure is converted to ferrite + perlite, by means of controlled cooling. When implementing this method, the strength and toughness of the microstructure can be ensured up to a certain level. However, when using this method, the effect of reducing production costs due to the absence of heat treatment disappears due to the introduction of a large number of alloying elements and obtaining sufficient strength becomes impossible due to the fact that the content of the ferrite fraction is low and the microstructure is enlarged. There is also a limitation that the viscosity of the microstructure is insufficient for intensive processing due to the formation of coarse-grained perlite.

In addition, despite the fact that a significant study of non-tempered grades of steel has been carried out to date, from the point of view of cost reduction due to the absence of heat treatment, the use of non-tempered steel obtained by this method, despite modern facilitation and hardening of vehicles, is undesirable for stamping to obtain a complex shape of the part in order to reduce the number of parts. Therefore, to overcome such limitations, a material is required that does not collapse and is able to sufficiently maintain the strength of the finished product even when processing involving severe deformation.

As a method that meets these requirements, the proposed method may include reducing the grain size of the material. In other words, strength and toughness, as somewhat conflicting requirements, can be satisfied simultaneously, since strength and toughness can be improved compared to conventional levels by reducing the grain size of the material.

An example of such a method is a method for reducing austenite grain size by reducing the C content and introducing a trace amount of Ti. In other words, according to this method, the content of C, which increases the resistance to deformation of the material, is reduced, and the decrease in strength due to a decrease in the content of C is compensated by a decrease in grain size. This method includes processing to reduce grain size, contributing to the so-called "fixing effect", according to which austenitic grains are fixed by inclusions based on Ti (for example, nitrides or carbonitrides) as a result of the introduction of Ti. This is because the strength and toughness of the material can improve after reducing the size of its grains, so that the material can be easily processed without destruction, even with the same deformation resistance and the force applied to it.

Elements having an effect, like Ti, can be Nb, V and the like. These elements also form inclusions, such as carbides or carbonitrides, and can affect the reduction in grain size of the material.

However, these elements have limitations in that they do not have enough effect to achieve the purpose of their introduction described below. In other words, as shown in FIG. 1, these elements have very high melting points of 1350 ° C or more, so these elements do not completely melt during re-heating of the slab, or the temperature of the formation of inclusions in most cases is limited to 900 ° C or less while the driving force for inclusion in the form of inclusions is relatively weak. In general, rolling is carried out within the temperature range of 850 ° C to 1050 ° C, and inclusion in the form of inclusions within the narrow temperature range is not effective, since it is desirable to continuously form inclusions within the rolling temperature range to reduce grain size .

Disclosure of invention

Technical challenge

In one aspect, the present invention relates to a thin steel bar comprising a simple component system that effectively reduces grain size due to the formation of inclusions within a wide temperature range, as well as a method for manufacturing a thin steel bar.

Technical solution

According to one aspect of the present invention, a thin steel bar is provided having a composition comprising: aluminum (Al) in an amount of about 0.07% by weight. to 0.14% by weight, and nitrogen (N), in which Al: N (where Al and N are% by weight of each element) ranges from about 15: 1 to about 25: 1.

In this case, the total content of Ti, Nb and V can be about 0.01% by weight. or less.

The thin steel bar according to the present invention may further comprise from about 0.15% by weight. up to about 0.3% weight. carbon (C), from about 0.05% weight. up to about 0.15% weight. silicon (Si), from about 1.0% weight. up to about 3.0% weight. Manganese (Mn), approximately 0.02% by weight. or less phosphorus (P) or about 0.02% by weight. or less sulfur (S).

AlN-based disperse nanoinclusions having a size of approximately 130 nm or less can be formed in a thin steel bar.

The thin steel bar may also have a microstructure comprising ferrite having an area fraction of approximately 50% to 70% and perlite having an area fraction of approximately 30% to 50%.

In addition, the thin steel bar may have a tensile strength range of approximately 600 MPa to 700 MPa, and an elongation range of approximately 20% to approximately 30%.

According to another aspect of the present invention, a method for manufacturing a thin steel bar is provided, comprising: finishing rolling a steel having said composition within a temperature range of from about 850 ° C to 1050 ° C; and cooling the steel at a cooling rate of approximately 5 ° C./sec or less.

The composition of the steel may further include from about 0.15% weight. up to about 0.3% weight. C, from about 0.05% weight. up to about 0.15% weight. Si, from about 1.0% weight. up to about 3.0% weight. Mn, about 0.02% weight. or less than P, or about 0.02% weight. or less S.

Advantages of the Invention

According to the present invention, a thin steel bar is developed that can be subjected to processing, such as cold stamping, without additional heat treatment, since a thin steel bar having sufficiently improved strength and toughness can be obtained with a simple alloying composition.

Description of drawings

The above and other aspects, features and other advantages of the present invention will become more apparent from the following detailed description in combination with the accompanying drawings, in which:

Figure 1 is a graph showing the temperature of formation and the equilibrium constant of inclusions that can form in steel;

Figure 2 is an optical micrograph showing the internal microstructure of each grade of steel made according to an example illustrating the present invention;

Figure 3 is a graph comparing the toughness of each sample made according to an example illustrating the present invention;

Figure 4 is a graph comparing the size of the austenite grains of each sample made according to an example illustrating the present invention;

Figure 5 is a micrograph of a thin rod taken under a transmission electron microscope, made according to an example illustrating the present invention;

6 includes four micrographs taken under a transmission electron microscope, confirming the distribution of inclusions in a thin rod made according to an example illustrating the present invention;

Fig. 7 is an optical micrograph showing internal microstructures when drawing and fabricating parts from a thin rod made according to an example illustrating the present invention; and

Fig is a graph showing the results of changes in grain size and toughness at room temperature according to changes in Al / N.

Preferred option

Further, the present invention is described in detail.

The inventors of the present invention have recognized the limitations in the art and have conducted in-depth studies to overcome technical limitations in the art. As a result, it was found that with the introduction of aluminum (Al) in a larger amount than usual, Al forms inclusions at most of the rolling temperatures. Therefore, during rolling, fusion centers are constantly created, as well as nucleation centers for the new microstructure.

When this process is carried out, austenite grains are crushed, as a result of which the grain size of the final microstructure, such as ferrite or perlite, obtained from the above microstructure is smaller than the grain size of a conventional microstructure. Therefore, this process effectively improves the strength and toughness of steel.

According to the present invention, in order to obtain a beneficial effect, the Al content is limited to a value of 0.07% by weight. up to 0.14% weight. The Al content is greater than the Al content typically introduced for deoxidation, and is within the desired range for the formation of Al-based inclusions (in particular nitrides) in steel. Therefore, to form a sufficient amount of Al nitrides, an amount of up to 0.07% by weight can be added. or more, up to 0.075% by weight. or more, up to 0.08% by weight. or more. However, if the Al content is too high, the proportion of oxide-based inclusions increases and the size of the inclusions increases so that Al not only does not contribute to grain size reduction, but also worsens the fatigue characteristics of the steel bar. Therefore, the upper limit of the Al content is 0.14% by weight.

Moreover, since the introduced Al forms dispersed inclusions having a nanosize, mainly interacting with nitrogen, the Al content and nitrogen content can have a quantitative ratio facilitating the thermodynamic formation of nitrides. If the nitrogen content is insufficient, nitrides do not form even with the introduction of a large amount of Al, and it may be possible that the resulting amount of oxides may become too large. On the other hand, if the nitrogen content is too high, there may be the possibility of the formation of angular cracks or during the casting a steel pouring cup may overgrow. In view of the foregoing, the Al: N ratio (where Al and N mean% wt. Of each element) may be from 15: 1 to 25: 1. The nitrogen content in the steel can be within any possible range, provided that it satisfies this condition. However, the nitrogen content, for example, may be from 0.0035 to 0.008% by weight.

In addition, microalloying elements, such as Nb, V, and Ti, typically introduced to reduce grain size, are essentially not added according to the present invention. However, since these elements enter the steel in various ways and are inevitably present, not being completely removed during the steelmaking processes, their total content may be 0.01% by weight. or less, for example, 0.005% by weight. or less. This is because the physical properties of the steel may deteriorate with the introduction of these elements.

In other words, according to the research results obtained by the inventors of the present invention, TiN is first released as a result of the interaction of nitrogen in the molten steel with Ti by co-introduction of Ti under conditions under which Al is present within the content range. Therefore, the grain size reduction effect contemplated by the present invention is hindered because the formation of AlN inclusions is prevented. The addition of Nb together with Al also contributes to the formation of angular cracks, and with the introduction of a large amount of Al, as in the present invention, the inclusions can be large. In addition, it is difficult to reduce the grain size due to large V (C, N) primary inclusions that form with the introduction of V.

The content of other elements is not particularly limited, with the exception of the content of Al, Nb, V or Ti. In other words, since the main distinguishing feature of the present invention is the reduction of grain size through the formation of dispersed inclusions as a result of the introduction of a large amount of Al, the introduction or exclusion of other components that do not affect the reduction in grain size can be easily carried out by a person skilled in the art. Therefore, their types and ranges may not be specifically limited.

However, since one of the main uses of the high-strength, high-viscosity steel manufactured according to the present invention is cold stamping, a steel component system is proposed as steel for the cold stamping described below, providing a more desirable effect by combining conditions for its components.

C: 0.15 to 0.3% by weight

C is an element substantially added to provide strength to steel. C may be contained in an amount of 0.15% by weight. or more, to obtain sufficient strength, while the content of C can be limited to 0.3% weight. or less to obtain sufficient viscosity. In particular, the C content may be limited to a range of 0.15% by weight. up to 0.3% by weight, taking into account the characteristics of the components of the present invention, steel components having a relatively high Mn content compared to the steel stamping coil described below.

Si: from 0.05% weight. up to 0.15% weight.

Si has the effect of enhancing the strength of the matrix, dissolving in ferrites. The Si content may be 0.05% by weight. to provide action for hardening the solid solution by Si. However, if the Si content is too high, machinability and toughness may be impaired due to the fact that the strain hardening is excessive during the cold stamping process. Therefore, the Si content can be adjusted at a level of 0.15% or less.

Mn: from 1.0% weight. up to 3.0% weight.

Mn is an alloying element that increases the strength of steel, affects impact properties, improves rolling properties and reduces brittleness. In particular, according to an illustrative embodiment of the present invention, the composition of the steel bar is designed to compensate for the decrease in strength due to Mn while reducing the content of C to provide viscosity. Therefore, Mn can be introduced in an amount of 1.0% by weight. or more. However, the Mn content may be limited to 3.0% by weight. or less, since strain hardening is strong when the Mn content is too high.

P: 0.02% weight. or less

The content of P can be adjusted at a level of 0.02% weight. or less, since P segregates at the grain boundaries and causes a deterioration in viscosity. The content of P can be adjusted at a level of 0.01% by weight. or less if adjusting the content of P during the grain size reduction process is possible.

S: 0.02% weight. or less

The S content can be adjusted as tightly as possible, since S, as a low melting point element, combines with Mn, reducing viscosity and adversely affecting the performance of a high-strength thin bar. However, the upper limit of the S content is limited to 0.02% by weight. taking into account the load during the process of reducing the grain size.

In other words, the composition of the thin rod according to the present invention may include, for example, a system of components in which the content of each element is controlled within ranges from 0.15 to 0.3% by weight. C, from 0.05 to 0.15% by weight. Si, from 1.0 to 3.0% by weight. Mn, 0.02% by weight. or less than P, 0.02% by weight. or less than S, in addition to a composition in which the contents of Al and N are controlled within the indicated ranges and Nb, V and Ti are essentially not added. In other words, a high-strength, high-viscosity thin bar can be obtained using a simple system of alloying components.

In contrast to the commonly proposed component system, into which a large number of alloying elements are introduced, the composition according to the present invention is a very simple component system, in which the corresponding conditions for the efficient formation of inclusions with Al are proposed, requiring the addition of only a few elements without introducing expensive alloying elements. Therefore, the cost of the alloy is lower, as well as the action in which there is not necessarily a difficult task, such as the regulation of components during steelmaking processes.

A thin steel bar having a composition according to the present invention can also include fine grains without being subjected to the usual complex rolling process to produce non-tempered steel by forming a large number of dispersed Al-based inclusions therein.

In other words, dispersed nanoinclusions based on AlN, the size of which is 130 nm or less, are formed in a thin steel bar having the indicated composition during the rolling and cooling processes. Since, as shown in FIG. 1, the formation temperature of such inclusions is approximately 850 ° C to 1050 ° C, the inclusions are highly dispersed and distributed in a thin steel bar during normal rolling (rolling is usually carried out within the specified temperature range), and , thus, to a large extent contribute to the reduction of the size of the internal grains. The smaller the size of the nanoinclusions, the more the nanoinclusions are dispersed and, thus, grain growth is more effectively prevented. Therefore, there is no need to specifically limit the lower size limit of Al-based inclusions. However, it can be concluded that the size of the inclusions is 10 nm or more under the indicated conditions for the formation of nitrides. In particular, it can be concluded that inclusions from 10 nm to 60 nm in size comprise approximately 20% to 30% of all inclusions by area, inclusions larger than 60 nm to 80 nm occupy approximately 40% to 50% of the area , and inclusions larger than 80 nm occupy the rest.

When inclusions are formed under these conditions, rolling is performed, and the bar is slowly cooled for subsequent cold rolling, while in a thin steel bar, a microstructure consisting of fine-grained ferrite of 15 μm to 40 μm (e.g., from 15 μm to 20 μm) can be obtained, and fine-grained perlite 20 microns - 55 microns (for example, from 20 microns to 25 microns). To obtain sufficient cold malleability, the portion of the ferrite portion in the microstructure may be approximately 50% to 70%, and the proportion of the perlite portion may be approximately 30% to 50%. A thin bar with an appropriate microstructure according to the present invention has a tensile strength range of approximately 600 MPa to 700 MPa, and an elongation range of approximately 20% to 30%, and an impact strength of 140 J or more (130 J or more for a drawn bar), without spheroidizing heat treatment. A drawn bar, which is obtained by drawing a thin bar with a range of compression ratio, comprising from 15% to 40%, i.e. the range of the degree of compression during conventional drawing, may also have a range of ultimate tensile strength, comprising from 800 MPa to 950 MPa, and an elongation range of from 15% to 25%.

The thin steel bar according to the present invention can be easily manufactured by any person skilled in the art, therefore there is no need for a specific limitation of the manufacturing method. Since Al-based dispersed inclusions can be formed within a wide temperature range as a hallmark of a thin steel bar according to the present invention, the applicable rolling temperature range is wide. Therefore, the cooling rate may vary in accordance with the microstructure properties necessary for a certain type of use of a steel bar.

In particular, if the thin steel bar according to the present invention is intended for cold stamping, there is no need for controlled rolling of steel in a narrow temperature range and cooling at a cooling rate having an adjustable range, as in the usual case. Even when rolling within the temperature range of 850 ° C to 1050 ° C, i.e. the usual rolling temperature range, and further cooling at a speed of 0.1 ° C / s to 5 ° C / s, often used to make a conventional steel bar for cold stamping, a thin steel bar can be obtained in which a sufficient amount is formed small grains.

Since inclusions are formed within the temperature range, i.e. from 850 ° C to 1050 ° C, in the present invention, a sufficient effect at the grain boundaries due to inclusions can be obtained when the finish rolling temperature is within the specified temperature range. Therefore, the finish rolling temperature can be selected as the temperature range.

As for the steel bar for cold stamping, since the microstructure consisting of ferrite and perlite is preferred, so that the bar does not have too high deformation resistance, slow cooling can be carried out at a speed of 5 ° C / s or less to the temperature Ar1 or less. Slow cooling of a conventional steel bar can produce undesirable results as the grain size becomes too large. On the other hand, with regard to the steel bar according to the present invention, fine grains can be formed even by slow cooling within the indicated range, since a large number of fine-grained inclusions are dispersed and distributed therein. However, when cooling at too low a speed, separate equipment may be required. Therefore, the cooling rate may be 0.1 ° C / s or more.

An embodiment of the invention

Further, the present invention is described in more detail in accordance with the accompanying drawings and the example described below. However, it should be noted that the example just illustrates the present invention, and does not limit its scope. Therefore, the scope of the present invention is determined by the appended claims and the details that follow logically.

Example

In order to understand the effect of the inclusions present in the examples, the austenite grain size (AGS) and toughness at 950 ° C are compared with a content of C, Si, Mn, P and S, adjustable at a level of 0.25% wt., 0.15% weight., 2.0% weight., 0.015% weight. and 0.016% by weight, changing the contents of Ti, Nb, V, Al and N according to the table below. To simulate the rolling conditions, the samples are subjected to solid solution treatment by heating them at 1180 ° C and then subjecting them to a strain of 0.6 (n ?) with a strain rate of 10 / s at 950 ° C.

Then the samples are rapidly cooled to room temperature to confirm the size of the austenite grains. The results are presented in table 1.

Table 1 Sample Ti Nb V Al N AGS (μm) V-shaped notch (J) U-shaped notch (J) Notes Sample 1 - - - - 0.004 32.6 140.34 156.45 Comparative standard metal Sample 2 - - - 0.04 0.004 15.8 165.89 180.63 The ratio of Al / N = 10 Sample 3 - - - 0.08 0.004 12,4 169.62 213.72 Al / N ratio = 20 Sample 4 0.015 - - - 0.004 17.1 160.3 175.89 Low Ti (C, N) Sample 5 0,030 - - - 0.004 18.3 79.48 130.22 High Ti (C, N) Sample 6 - 0,03 - - 0.004 32.6 93.49 132.13 Low Nb (C, N) Sample 7 - 0.06 - - 0.004 23.8 156.8 160,5 High Nb (C, N) Sample 8 0.015 0,03 - - 0.004 28.1 139.9 166.97 Co-administration of Ti and Nb Sample 9 - - 0.05 - 0.004 20.6 103.65 177.15 Low V (C, N) Sample 10 - - 0.01 - 0.004 18.7 30.86 67.96 High V (C, N)

From table 1 and figure 2, it is obvious that in example 3, according to which only aluminum and nitrogen were introduced, but not Nb, V and Ti were introduced, as in the present invention, the austenite grain size (AGS) at 950 ° C was about 12.4 microns, which is very small compared to other samples. In the event that the grain size at 950 ° C. is small, microstructures such as ferrites and perlites formed in thin austenites also have a small grain size.

As shown in table 1 and figure 3, it is also clear that the toughness (V-shaped notch, U-shaped notch) of sample 3 having a small austenitic grain size is the best.

Samples having the same composition as the samples described in table 1 are subjected to solid solution treatment at 1180 ° C, and then subjected to deformation (drawing) of 0.6 with a strain rate of 10 / sec at 800 ° C, 850 ° C, 900 ° C and 950 ° C, respectively. Then the samples are rapidly cooled; the results of comparing the size of their austenite grains are presented in figure 4. From figure 4 it is obvious that small grain sizes were obtained under all temperature conditions of example 3, corresponding to the conditions of the present invention.

When the deformation is carried out at 800 ° C, which does not correspond to the rolling temperature of a conventional thin steel bar for cold stamping, the size of austenite grains in all cases is 30 μm or more. This allows us to indirectly suggest that rolling to obtain small grains can be carried out at a temperature of 850 ° C or more.

Micrographs of steel under a transmission electron microscope, made under the same conditions as sample 3 in table 1, are presented in figure 5. As shown in FIG. 5, it can be concluded that cubic AlN, the size of which is about 130 nm or less, is distributed finely and evenly in the steel.

To determine the particle size distribution of dispersed inclusions, micrographs were also taken under a transmission electron microscope of four sections of sample 3, showing the distribution of inclusions, including the result shown in Fig. 5, after which the particle size distribution of each inclusion was determined. The results are presented in Fig.6. Based on these results, it can be concluded that the fraction of the inclusion site, whose size is from 10 nm to 60 nm, is 29.2%; the fraction of the inclusion site, the size of which is from more than 60 nm to 80 nm, is 48.6%; and the fraction of the inclusion region, the size of which is from more than 80 nm to 130 nm, is 22.2%. The scale in FIG. 6 is 0.2 μm.

Next, Fig. 7 shows optical micrographs of samples, the steel of which, manufactured under the same conditions as sample 3, is drawn and then cold stamped. As shown in Fig. 7, based on these samples, it can be concluded that the content of fine-grained ferrite, with a size of 15 μm to 20 μm, is approximately 65% to 70%, and the content of perlite with a size of 20 μm to 25 microns, approximately 30% to 35%. According to the results of measurements of the magnitude of the impact of the samples, it can be concluded that the samples with a V-shaped notch and a U-shaped notch have high impact values ranging from 55 J to 60 J and from 150 J to 190 J, respectively. Therefore, it can be concluded that a non-tempered thin steel bar having sufficient machinability can be made by adjusting the composition of the thin steel bar and rolling according to the conditions of the present invention.

To confirm the fact that sufficient strength and toughness can be obtained in the manufacture of steel having the desired state, as a result of this rolling process of a thin bar, a blank in the form of a thin rod having the same composition as sample 3 satisfying the desired composition according to the present invention. To obtain a thin bar, the billet (billet) is heated at 1150 ° C, rough rolling and finishing rolling are carried out at 910 ° C and 1050 ° C, respectively, and calibration rolling is carried out at 1035 ° C. The rolled thin bar is cooled at a speed of 0.5 ° C / sec, and then coiled at 835 ° C. The bar is cooled to 500 ° C with a cooling rate of 0.5 ° C / sec, whereby after cooling with air, a bar with a diameter of 18 mm is obtained (Example 1 illustrating the present invention). The characteristics of the cold drawn rod obtained by drawing with a degree of reduction of 28.2% are determined (example 2 illustrating the present invention).

The characteristics of a thin bar (comparative example 1) and a cold drawn thin bar (comparative example 2) are also determined, respectively obtained by the methods described in examples 1 and 2 illustrating the present invention, with the addition of 0.015% weight. Ti instead of Al, as well as a wire rod (comparative example 3) and a cold drawn thin rod (comparative example 4), respectively obtained by the methods described in examples 1 and 2 illustrating the present invention, with the addition of 0.01% weight. V instead of Al, in which the content of C, Si, Mn, P, S, and N is the same as in Example 1 illustrating the present invention. The results are presented in table 2.

table 2 Example YS yield strength (MPa) TS tensile strength (MPa) EI elongation (%) R.A. contraction (%) U-shaped notch (J) V-shaped notch (J) Example 1 illustrating the invention 378 658 24.0 61,4 150.7 135 Example 2 illustrating the invention 757 835 18,4 60.3 143 98 Comparative Example 1 320 580 19,2 55.7 120.3 100.1 Reference Example 2 702 789 15,5 53.1 97.7 83.1 Reference Example 3 333 592 18.9 57.2 121.7 103.1 Reference Example 4 711 797 16,2 56.6 95.1 80.7

As shown in table 2, in the examples illustrating the present invention, the tensile strength (TS) of the thin rod is 658 MPa or more, and after drawing, the tensile strength increases to approximately 835 MPa or more. However, with respect to comparative examples, the tensile strength of the rods is 580 MPa (comparative example 1) and 592 MPa (comparative example 3), which is 60 MPa or more less than the tensile strength in example 1 illustrating the present invention, while the limit tensile strength is only about 789 MPa (comparative example 2) and 797 MPa (comparative example 4) after drawing, which is approximately 40 MPa less than the tensile strength in example 2, illustrating the present invention.

The amount of elongation, which is an indicator of the machinability of a non-tempered thin steel bar in Example 1, illustrating the present invention, i.e. the example of a thin bar according to the present invention is also high, approaching 24%, however, the elongation values in comparative examples 1 and 3, in which the introduction of Ti, V or the like, contributes to a decrease in grain size, are 19.2% and 18, respectively , 9%, which is approximately 5% less than in example 1, illustrating the present invention. This difference is constantly maintained even after drawing, so, in example 2 illustrating the present invention, the elongation is 2% or more higher than the same values in comparative examples 2 and 4. Therefore, it can be concluded that when adding a large amount Al improves both strength and machinability, and the Al: N ratio rises to about 20: 1.

It can also be concluded that the constriction values (R.A.) in the examples illustrating the present invention are about 3-6% higher than the same values in the comparative examples.

In addition, it can be concluded that the toughness values of samples with a U-shaped notch and a V-shaped notch in the examples illustrating the present invention, which are viscosity indices, are 25 J or more higher than the same values in the comparative examples. Thus, it can be confirmed that the thin steel bar made according to the present invention also has a high level of viscosity.

Therefore, it can be concluded that a thin steel bar made according to the conditions of the present invention can be used to obtain products requiring high strength and high viscosity, such as a cold stamping bar.

In order to check the effect of the Al / N ratio on the grain size and impact value at room temperature, the grain size and impact value (V-notch test) were studied with varying the Al / N ratio by changing only the N content, while other conditions remain so same as in example 3; the results are presented in Fig. 8. In Fig. 8, grain size units and viscosity are indicated in μm and J, respectively. As shown in FIG. 8, the impact value (i.e., the absorption energy upon impact) at room temperature has a low value of 110 J or less, while the Al / N ratio is 5, however, the impact value increases sharply with Al / N equal to 10 or more. The magnitude of the impact also decreases sharply with an Al / N ratio of more than 25, and the magnitude of the impact with an Al / N ratio of 30 is the same as the magnitude of the impact with an Al / N ratio of 5. Therefore, it can be concluded that that when taking into account the magnitude of the impact, the Al / N ratio can be from 10 to 25. However, it was confirmed that with an Al / N ratio of 10, the grain size is 35 μm or more, and the effect of reducing the grain size is weak At the same time, frequent occurrence of angular cracks during casting is also observed. Therefore, it can be concluded that the Al / N ratio can be from 15 to 25.

Despite the fact that the present invention has been shown and described in connection with illustrative options for its implementation, specialists in the art will understand that modifications and deviations are possible without violating the essence and scope of the present invention defined by the attached claims.

Claims (7)

1. A high strength, high viscosity thin steel bar comprising: aluminum (A1) in an amount of from about 0.07 wt.% To about 0.14 wt.%, And
nitrogen (N)
moreover, Al: N, where Al and N mean weight% of each element, is from about 15: 1 to 25: 1, while dispersed AlN-based nanoinclusions having a size of about 130 nm or less are formed in the bar. 2. The bar according to claim 1, in which the total content of Ti, Nb and V is approximately 0.01 wt.% Or less. 3. The bar according to claim 1 or 2, further comprising from about 0.15 wt.% To about 0.3 wt.% Carbon (C), from about 0.05 wt.% To about 0.15 wt.% Silicon (Si), from about 1.0 wt.% To about 3.0 wt.% Manganese (Mn), about 0.02 wt.% Or less phosphorus (P) or about 0.02 wt.% Or less sulfur ( S). 4. The rod according to claim 1 or 2, which has a microstructure comprising ferrite having an area ratio of approximately 50% to 70% and perlite having an area ratio of approximately 30% to 50%. 5. The bar according to claim 1 or 2, in which the range of tensile strength is from about 600 MPa to 700 MPa and the elongation range is from about 20% to 30%. 6. A method of manufacturing a high strength, high viscosity thin steel bar, including:
fine rolling of steel having a composition containing aluminum (Al) in an amount of from about 0.07 wt.% to about 0.14 wt.% and nitrogen (N), wherein Al: N, where Al and N mean weight. % of each element is from about 15: 1 to 25: 1, within the temperature range of approximately 850 ° C to 1050 ° C; and
cooling the steel at a rate of approximately 5 ° C / s or less to obtain a bar in which dispersed AlN-based nanoinclusions are formed having a size of approximately 130 nm or less. 7. The method according to claim 6, in which the steel further comprises from about 0.15 wt.% To about 0.3 wt.% C, from about 0.05 wt.% To about 0.15 wt.% Si, from about 1.0 wt.% to about 3.0 wt.% Mn, about 0.02 wt.% or less P, or about 0.02 wt.% or less S.

Images